Matrix metalloproteinases (MMPs) are a family of zinc-dependent endoproteinases whose enzymatic activity is directed against components of the extracellular matrix (ECM). MMPs facilitate tumor cell invasion and metastasis by at least three distinct mechanisms.

When carbon dioxide levels fall as the blood passes through the lungs, bicarbonate levels fall in the serum because the equilibrium shifts to replace CO2, and consequently bicarbonate in the red blood cells will move out into the serum. To balance the charges when bicarbonate exits the cell, a chloride anion from the plasma enters the red blood cell.